Seattle DOT Adds Immersed Tunnel Option to RFQ

 

Much like the process itself, the proposal of replacing the West Seattle Bridge with an immersed tunnel has slipped into the Request for Qualification (RFQ). With residents supporting the idea of a shallow tunnel, which would carry light rail, the Seattle Department of Transportation changed the contract solicitation to include the tunnel as an option.

Engineer companies will bid for a contract valued at up to $150 million to design and supervise construction of a bridge replacement. The West Seattle Bridge has been closed since late March due to shear cracks in the concrete girders. The cracks were getting worse on the bridge, which carries the most traffic of any spans in the city.

Retired engineer Bob Ortblad suggested the immersed tunnel idea, which involves sliding precast concrete sections underground. Ortblad’s idea does come with challenges. Digging trenches on Harbor Island for cars to enter the tunnel is one of them. West of the Duwamish Waterway lies another hurdle where geometry is particularly tight for traffic exiting the tunnel.

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